A horizontal oscillator of a television receiver is controlled to be in synchronism with the horizontal sync signal in a video signal. The horizontal oscillator is often a voltage controlled oscillator or VCO which provides a horizontal signal in phase with the sync signal. A phase comparator having an output coupled to the VCO compares the sync signal obtained from a sync separator with a horizontal output signal derived from the horizontal deflection circuit. The phase comparator includes a current output that charges or discharges a capacitor of a low pass filter to generate a DC control voltage for the VCO such that the frequency is increased or decreased within an available range, thereby keeping horizontal deflection in synchronism with the received horizontal sync.
During certain states of operation of the television receiver the sync signal may be missing or undependable. The sync signal may be missing or undependable when, for example, the tuner is set to a frequency that has no transmitted signal, when the signal is noisy or very weak, or when the transmitted signal has been scrambled to remove or reposition the sync.
It is known, that during the certain operational states where the sync signal may be missing or unusable, it is desirable prevent the anomalous triggering or mis-synchronisation of the horizontal oscillator. Such methods generally function by inhibiting the coupling of a composite video signal to a synchronizing pulse separating means. With the input to a sync separator inhibited, the output may well assume either the supply voltage or ground potential. This output will be coupled to a input of a phase comparator which controls the horizontal oscillator. Hence, with the separated sync input inhibited, the oscillator will tend to operate at a nominal frequency determined by the output voltage from the phase comparator. It is also known that it is desirable to prevent the horizontal oscillator from assuming a lower than nominal frequency to avoid activation of over voltage protection.
A control output of a microprocessor may be used to block transmission of the signal from the sync separator to the phase comparator in order to ignore the output of the sync separator. The voltage controlled oscillator is then allowed to free run. Where the voltage controlled oscillator is crystal controlled, e.g., with a ceramic resonant oscillator, merely decoupling poor sync from the sync separator from the phase comparator may be inadequate to ensure stable operation of the horizontal oscillator in view of the high internal loop gain of such oscillators. Internally generated noise in the phase detector can provide spurious signals which cause the frequency of the horizontal oscillator to shift irregularly.
An on-screen display (OSD) may be caused to jitter or become ragged or vary in size or position during conditions where the sync signal is intermittent or unreliable. In extreme cases the on-screen display becomes unreadable because the circuitry for detecting the sync is mis-triggered or triggered erratically with the consequence that the scanning frequencies are varied irregularly, with the result that the on-screen characters are incorrectly read out.